The development of the erbium-doped fiber devices (EDFDs) has greatly impacted optical fiber communications. This is because EDFDs generate light efficiently in the 1530-1560 nm low-loss wavelength range for silica fibers. For the most part, new high capacity systems operate within this range at a nominal wavelength of 1550 nm.
The recent deployment of WDM optical fiber systems is due in large part to the EDFD. WDM optical fiber systems maintain span distances of hundreds of kilometers, eliminating the need for optical-to-electronic repeaters while at the same time increasing capacity by increasing the number of WDM channels. The amplification band of EDFDs is sufficiently broad to simultaneously amplify multiple channels of a WDM set. For example a typical EDFD with an amplification band about 30 nm wide is adequate for operating a 40 channel system, with channels 0.8 nm apart.
Light-generating devices which operate efficiently over a variety of infrared frequencies have potential for increasing the number of channels that can be operated in a WDM optical fiber system. Systems employing such a light-generating device would be capable of exploiting the newly broadened wavelength bandwidth (.about.1210 nm-1700 nm) of more recent optical fibers. These fibers exhibit another useful low-loss wavelength range centered at about 1310 nm. Unfortunately, such a light-generating device has not been developed. Materials based on Nd.sup.+3 /fluoride glass and Pr.sup.+3 /ZBLAN have received serious attention for use at 1310 nm, but have limitations such as requirements for high pump powers.
Light-generating devices made from crystalline materials have many well known advantages over devices made from glass. These advantages include avoiding the radiationless transition specific to glass fibers, smaller size, and suitability for integration on a substrate. U.S. Pat. No. 5,535,051 describes a WDM optical fiber system using a crystal optical amplifier. The amplifier uses a disordered, crystalline host to broaden the useful range of narrow-band active dopants such as erbium and praseodymium. Unfortunately, this amplifier is not capable of operating efficiently at 1310 nm.
Accordingly, there remains a need for a multi-wavelength optical fiber system employing an improved wideband light generating device.